Inductive DC to DC converters are well known in the art of electronic power supplies. Such converters operate by using an electronic switch to pass a current through an inductor and then interrupt the current periodically to produce a "flyback" voltage for transfer through a diode to a capacitive load. These converters are especially useful in battery powered equipment, such as portable communication receivers, in which most elements of the equipment can operate from a low voltage, e.g., 1.5 volt, power supply, but one or more of the elements require a substantially higher DC or AC supply voltage.
Because battery life is normally of great concern in battery powered equipment, inductive DC to DC converters must be designed to operate efficiently. A conventional method of improving the efficiency of such converters is to utilize a peak coil current control circuit to prevent the current through the inductor from exceeding a predetermined level beyond which the magnetic core of the inductor would saturate, thus rendering it substantially impossible to store additional energy in the inductor.
Even with the utilization of the peak coil current control circuit, the conventional inductive DC to DC converter can lose efficiency because the timing of each periodic transfer of stored energy from the inductor to capacitive load is not precisely controlled. For greatest efficiency, the ideal operating cycle would first transfer all of the stored energy from the inductor and then would begin immediately recharging the inductor with energy. Unfortunately, variations in battery voltage, coil inductance, parasitic capacitances and resistances, etc. cause the time required per ideal cycle to vary from one converter to the next, thus rendering it very difficult to generate the ideal cycle solely on the basis of timing.
To ensure that all of the energy stored in the inductor is transferred to the load during each cycle, the conventional converter allows an extra margin of time before beginning the next recharging cycle. Unfortunately, the extra margin of time causes a departure from the ideal cycle, thereby reducing efficiency. In addition, at the point of total energy transfer the diode coupling the inductor to the capacitive load becomes reverse biased, thereby presenting a high impedance to the inductor, which allows the inductor voltage to oscillate or "ring" with parasitic capacitances present in the electronic switch. Such oscillations can produce catastrophic effects in nearby circuit elements, e.g., turning on parasitic transistors, causing devices to latch, desensitizing a radio receiver, etc.
Thus, what is needed is an apparatus for maximizing the efficiency of an inductive DC to DC converter. It is highly desirable that the apparatus also prevent the potentially damaging oscillations encountered in the conventional converter.